The present invention relates to a switching power source apparatus with a power factor corrector and a DC-DC converter and to a power factor corrector. In particular, the present invention relates to a technique to reduce power consumption during a standby period of such a switching power source apparatus.
FIG. 1 shows a switching power source apparatus according to a related art. This apparatus has a power factor corrector (PFC) and a DC-DC converter connected to the PFC. The PFC conforms to input harmonic current regulations stipulated in IEC61000-3-2 recently established.
Operation of the apparatus shown in FIG. 1 will be explained. An AC power source 1 provides a sinusoidal voltage, which is passed through a filter 2 and is full-wave-rectified with a diode bridge 3. The full-wave-rectified voltage is passed through a filter 4 and is supplied to the PFC 5. The full-wave-rectified voltage is passed through a reactor L, a diode D1 and a starting resistor R1 for the PFC 5 and is used to charge a power source capacitor C3 for a PFC controller 6. At the same time, the full-wave-rectified voltage passed through the reactor L and diode D1 is supplied to a starting resistor R2 for the DC-DC converter and is used to charge a power source capacitor C2 for a DC-DC converter controller 7.
When the voltage of the power source capacitor C3 reaches a starting voltage of the PFC controller 6, the PFC controller 6 starts to turn on and off a switching element Q1. This ON/OFF operation is hereinafter referred to as “switching operation.” The switching operation drives the PFC 5.
The PFC 5 has the reactor L, switching element Q1, diode D1, output capacitor C1, and PFC controller 6 and serves as a step-up chopper. In the PFC 5, the full-wave-rectified voltage from the filter 4 is supplied to a series circuit consisting of the reactor L and switching element Q1. The PFC controller 6 turns on and off the switching element Q1, to correct the power factor of the AC power source 1. At the same time, the diode D1 and output capacitor C1 of the PFC 5 provide a DC voltage. The PFC controller 6 receives a voltage of the output capacitor C1 detected by a voltage detector (not shown), and according to the value of the voltage, controls the switching operation of the switching element Q1 so that the output capacitor C1 keeps a constant DC value at the terminals thereof.
The PFC controller 6 multiplies an error voltage, which is obtained by amplifying an error between the voltage of the output capacitor C1 and a reference voltage, by the full-wave-rectified voltage from the filter 4 and provides a multiplied output voltage. The switching element Q1 is connected in series to a current detecting resistor (not shown) that detects an input current. The PFC controller 6 finds an error between a voltage proportional to the detected input current and the multiplied output voltage, amplifies the error, and provides an error voltage. Thereafter, the PFC controller 6 generates a pulse signal that is, for example, ON if the error voltage is equal to or greater than the value of a triangular wave signal and is OFF if the error voltage is lower than the value of the triangular wave signal. The pulse signal is applied to the gate of the switching element Q1. This results in shaping, every half cycle, the input current passed to the current detecting resistor into a sinusoidal wave analogous to the shape of an input voltage to the AC power source 1, thereby correcting the power factor of the AC power source 1.
When the voltage of the power source capacitor C2 reaches a starting voltage of the DC-DC converter controller 7, the controller 7 starts the switching operation of a switching element Q2. When the switching element Q2 is turned ON, a current is passed to a primary winding P of a transformer T, and therefore, the transformer T accumulates energy. When the switching element Q2 is turned off, the energy accumulated in the transformer T is discharged as a voltage from a secondary winding S of the transformer T. The voltage is rectified and smoothed through a diode D5 and a smoothing capacitor C5 and is supplied as a first DC voltage Vo from an output terminal to a load RL.
The first DC voltage Vo supplied to the load RL is compared in a voltage detector 8 with a reference voltage (not shown). The voltage detector 8 provides an error signal representative of the difference between the first DC voltage Vo and the reference voltage to a light emitter 9a of a photocoupler. In response to the error signal, the light emitter 9a emits light. The light representative of the error signal is received by a photodetector 9b of the photocoupler and is transferred to the DC-DC converter controller 7. Based on the received signal, the converter controller 7 carries out PWM control to adjust an ON-time of the switching element Q2 so as to maintain the first DC voltage Vo at a constant value.
When the switching element Q2 is turned off, the energy accumulated in the transformer T is also discharged from a control winding C of the transformer T. The discharged energy is transferred through a diode D2 to the power source capacitor C2 and through a diode D3 to the power source capacitor C3. The voltage of the power source capacitors C2 and C3 serves as source power for the PFC controller 6 and DC-DC converter controller 7.
The switching power source apparatus of FIG. 1 detects a light- or no-load state according to a voltage drop at a current detecting resistor R3 on the secondary side of the transformer T. The light- or no-load state decreases a load current, thereby dropping a terminal voltage of the current detecting resistor R3. A current detector 10 compares the terminal voltage of the current detecting resistor R3 with a reference voltage (not shown), and if the difference between the terminal voltage and the reference voltage is equal to or lower than a predetermined value, makes a light emitter 11a of a photocoupler emit light. Then, a photodetector 11b of the photocoupler turns on to turn off the PFC controller 6 on the primary side of the transformer T.
In this way, the related art can stop the operation of the PFC 5 in a light- or no-load state, to reduce the switching loss of the PFC 5 as well as cutting power consumption during a standby period. The harmonic current regulations mentioned above do not regulate an input power of 75 W or below, and therefore, there is no problem of stopping the PFC 5 and is rather preferable from a worldwide trend of minimizing standby power.
A technique for stopping a power factor corrector (PFC) in a light- or no-load state is disclosed in Japanese Unexamined Patent Application Publication No. Hei-8-111975. According to the disclosure, a DC-DC converter detects a light-load state by utilizing a phenomenon that the voltage of a tertiary winding of a transformer decreases in the light-load state. Upon detecting a light-load state, the disclosure cuts an auxiliary power source for the PFC, to stop the PFC.